Low-pressure gas discharge lamps, such as compact fluorescent lamps, have a pair of tungsten cathodes which are coated with a metal oxide that emits electrons when heated. The electrons ionize pressurized gas within the lamp envelope, and when a high voltage is applied across the cathodes, electrical current is discharged between the cathodes in the form of a plasma arc that emits ultraviolet radiation. The interior of the lamp envelope is coated with a material that responds to the ultraviolet radiation by emitting visible light. The plasma arc within the lamp has a negative resistance characteristic that requires a series ballast impedance to maintain stable current flow.
The electron-emissive material gradually depletes through normal use of the lamp; consequently, the cathode current required to sustain the plasma arc increases substantially over a period of time. At the end of the useful life of the lamp, the power consumed by the depleted cathode may increase by a factor of ten or more, thus causing excessive heating of the cathode. Cathode overheating can cause the cathode to break and come into contact with the glass envelope, thus causing a local hot spot at extremely high temperatures approaching 2,000.degree. C. Such localized over-heating can be intense enough to cause the surrounding glass envelope to crack and melt the plastic body of the base. This condition can also occur at any point in the life of the lamp if the cathode should break, come into contact with the glass wall of the lamp and the ballast continues to supply enough energy to maintain the arc.
Localized over-heating can occur in low wattage, compact fluorescent lamps, for example fluorescent lamp types T2 and T4 corresponding with lamp envelope diameters of 1/4" and 1/2", respectively. The cathodes of such lamps are located close to the glass envelope as compared with larger diameter lamps, for example type T8 (one inch diameter). Because of the limited radical spacing between the cathode and the glass wall, the small diameter fluorescent lamp can be subjected to excessive local overheating and extremely high temperatures in response to excessive current flow through a short-circuit cathode or a depleted cathode that has broken and comes into contact with the glass wall of the lamp.
Moreover, the gas discharge lamp operates on alternating current with the current flowing through the lamp in both directions, between the two electrodes that act alternately as a cathode and as an anode, and vice versa. Under end-of-life operating conditions, one of the two electrodes will become depleted of the alkaline metal oxide coating, thus losing its ability to emit electrons relative to the other electrode. This will cause the lamp to conduct electric current more readily in one direction than the other, thus generating a substantial direct current (DC) component that can damage the ballast circuit. Also, the direct current component can saturate the magnetic core of an inductive ballast, thus causing it to lose control of the AC voltage and current applied to the lamp.